Added guard around import to avoid failing when walberla is there but no python module is built

pystencils/boundaries/boundaryhandling.py

@@ -8,11 +8,18 @@ from pystencils.boundaries.createindexlist import (
     create_boundary_index_array, numpy_data_type_for_boundary_object)
 from pystencils.cache import memorycache
 from pystencils.data_types import TypedSymbol, create_type
-from pystencils.datahandling import ParallelDataHandling
 from pystencils.datahandling.pycuda import PyCudaArrayHandler
 from pystencils.field import Field
 from pystencils.kernelparameters import FieldPointerSymbol
 
+try:
+    # noinspection PyPep8Naming
+    import waLBerla as wlb
+    if wlb.cpp_available:
+        from pystencils.datahandling.parallel_datahandling import ParallelDataHandling
+except ImportError:
+    ParallelDataHandling = None
+
 DEFAULT_FLAG_TYPE = np.uint32